The invention relates to the field of attitude determination of space vehicle. More particularly, the invention relates to the use of a patch antenna for attitude determination of a spinning space vehicle.
The spin axis attitude determination of a spinning space vehicle has been traditionally accomplished by using a combination of sun and earth horizon sensors. A sun sensor measures the sun illumination spike and the elevation angle with respect to the spin axis of the space vehicle. Sun position and elevation information establishes a geometric sun position cone with a half cone angle equal to the elevation angle on which the spin axis lies. A similar earth elevation cone can be constructed based on the earth sensor measurements. The intersection of the sun position cone and earth elevation cone provide the inertial attitude information for determining the spin axis of the rotating vehicle. The cost of such a system can be appreciable. It is desirable to find cost effective alternatives to the use of expensive sun and horizon sensors.
U.S. Pat. No. 4,414,550 issued Nov. 8, 1983 discloses one-quarter wave plate patch antennas. U.S. Pat. No. 5,450,090 discloses an elongated microstrip antenna. These patch and strip antennas are inexpensive to manufacture. A one-quarter wave plate patch antennas can be used for receiving transmitted RF signals. U.S. Pat. No. 4,962,383 issued Oct. 9, 1990 discloses an array antenna system using a plurality of xc2xc wave plate that couples a phase response from one patch antenna to another patch antenna as a pair for use as an element in an array antenna. The two patch antennas can radiate microwave signals in a forward and reverse direction with beam control. The antenna patterns of the two patch antenna are combined to form a single antenna beam. These patch antennas provide hemispherical antenna patterns that are used to receive incoming signals over a wide angular range. These patch antennas do not provide an ability for attitude determination of a spinning spacecraft.
Attitude determination of a space vehicle using GPS signals can be based on an interferrometry of a spatially separated multiple patch antenna configuration. Using the relative phase differential between two selected patches, an angular error with respect to an axis perpendicular to the line joining the two patches can be determined. However, the angular error is with respect to a stationary axis unsuitable for attitude determination of a rotating system. While patch antennas have been used for attitude determination of a stationary spin axis space vehicle, such patch antenna have not been used for attitude control in a rotating space vehicle. These and other disadvantages are solved or reduced using the invention.
An object of the invention is to provide a system and method for attitude determination in a rotating space vehicle.
Another object of the invention is to provide system and method for attitude determination of a rotating space vehicle using an antenna pattern having a null slit.
Yet another object of the invention is to provide system and method for attitude determination of a rotating space vehicle using a pair of patch antennas for forming an antenna pattern having a null slit.
Still another object of the invention is to provide system and method for spin axis attitude determination of a rotating space vehicle using a pair of patch antenna for forming an antenna pattern having a null slit for time referencing received GPS signals from GPS satellites.
Still another object of the invention is to provide system and method for determining the spin axis attitude of a rotating space vehicle using a pair of patch antenna for forming an antenna pattern having a null slit for receiving GPS signals from GPS satellites functioning as pseudo star references.
The invention is directed to the creation and use of an electronic nulling slit antenna pattern for determining the spin axis attitude of a spinning space vehicle. A preferred sensor system includes a GPS receiver, a conventional sum and difference hybrid, and two juxtaposed xc2xc wave patch antennas. The two xc2xc wave patch antennas are placed side by side, separated by an optimum distance, on a lateral surface of a rotating space vehicle. The side by side placement causes an overlapping of the two antenna patterns each of which having a hemispherical pattern having a peak gain at nearly the center position. A conventional hybrid is used to create sum and difference signals from the two patch antennas. The difference signal is created by the hybrid introducing a 180 degree phase shift between the two patches signal creating an electronic null slit, that is, a null in the difference antenna pattern along the plane of symmetry between the patch antennas. This plane of symmetry is referred to as the center plane position. The sum signal provides a peak gain at the center plane position between the antennas. The sum signal is used for conventional GPS signal detection and reception and for computing a navigation solution of the spinning space vehicle. The difference signal is used as a time reference among a plurality of received GPS signals for determining the spin rate of the spinning space vehicle. The time reference is then also used to determine azimuth angles for the GPS satellites for computing the spin axis attitude of the spinning vehicle.
The system and method use visible GPS satellites as pseudo stars. The sum and difference GPS signals are recorded when the signal to noise ratio exceeds a predetermined threshold indicating the presence of a GPS signal and hence the presence of a GPS satellite in the field of view of the patch antennas. The null slit pattern of the GPS difference signals is used for time correlation of all of the received GPS signals. The null slit is used to reference the received GPS signals to precise times when the GPS signals enter a narrow field of view of the null slit of the patch antenna difference pattern. The split dual patch antenna serves to track the GPS signals from all of the visible GPS satellites. Each of GPS sum and difference signals are recorded with time stamps for time cross-referencing and time correlation among all of the GPS sum and difference signals. The system and method are then used to determine the spin rate and spin axis attitude of the rotating vehicle from the recorded time correlated sum and difference GPS signals. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.